Multifocal lenses are known and widely used, being prescribed for those requiring different dioptric powers for distance and reading vision. This condition is called "presbyopia". It mostly appears when one gets older and age renders it difficult for the eye (if not impossible) to focus on near and distant objects. This condition may be relieved by prescribing one pair of lenses for distance vision and another for reading vision. A single pair of either bifocal or multifocal lenses can replace both of these pairs of lenses.
The most common multifocal lens is the so-called "progressive addition lens" or PAL. This lens has a far vision zone located at the top of the lens, and a near vision zone located at the bottom of the lens, the far and near vision zones being connected by an intermediate transition region. In this intermediate transition region, called "corridor", focus changes continuously from the focus at the far vision zone to that of the near vision zone. The locations of the far and near vision zones are dictated by the prescribed parameters of the optical powers required for an individual's "distance" and "reading" visions which define the lens' addition. Thus, a PAL is characterized by the gradual change of its optical power from the top of the lens to the bottom.
It is obvious that both the upper, far vision zone and the lower, near vision zone of the PAL should be as wide as possible, to enable the wearer to read comfortably, without having to move his head sideways to follow the text. However, large far and near vision zones result in a shorter intermediate transition region in between them, i.e. corridor, making this region less usable.
PALs are generally of two designs, so-called "hard" and "soft" designs, depending on the distance between the far and near vision zones, i.e., the length of the corridor, which ranges typically between 16 and 24 mm. The "hard" design is characterized by a relatively sharp transition region, as compared to that of the "soft" design. Such a sharp transition region induces large distortions on either side of the corridor. The "soft" design sacrifices the widths of far and near vision zones in favor of a more gradual transition with less distortion. The main principles of PALs of "hard" and "soft" designs are illustrated in FIGS. 1 and 2, respectively.
FIG. 1 shows an image I.sub.1 of a pattern of equidistant, vertical lines L, which is obtained through a PAL with "hard" power variations. It is evident, that the optical power at a bottom region R.sub.B of the lens, i.e. in the near vision zone which produces a relatively larger magnification for the reading vision, is larger than the power at a top region R.sub.T which is used for distance vision. Additionally, the line separation .DELTA.L.sub.B in the bottom region R.sub.B is wider than the line separation .DELTA.L.sub.T in the top region R.sub.T. With a progressive continuous power profile, top portions L.sub.T of the imaged lines L join smoothly with bottom portions L.sub.B, so that they must bend in the intermediate region, or corridor, of the lens. However, the line bending effect produces image distortion, for example, similar to that one would observe through a cylindrical lens with an axis at 45.degree.. As shown, the distortion is minimal along the central vertical line L.sub.0, and increases towards periphery regions at both sides of the central vertical line L.sub.0. The width of the intermediate region between the far and near vision zones of the lens is determined by an area in the lens where distortion remains below a predefined threshold, normally 0.5 diopter cylinder. Thus, in the PAL of the "hard" design, the corridor is both short and narrow.
FIG. 2 illustrates an image I.sub.2 of a pattern of equidistant, vertical lines L, which is obtained through a PAL of a "soft" design with much more gradual progression. It is seen that the line bending is less pronounced here, as compared to that of the hard PAL shown in FIG. 1. Moreover, the distortion is lower, and a corridor is longer and wider. The height of a near vision zone, through which a lower region R.sub.B of the image I.sub.2 is obtained, and which is defined by the region where line-portions L.sub.B are straight and equidistant, is nevertheless much shorter. The same may be said about a far vision zone through which the upper region R.sub.T of the image I.sub.2 is obtained.
Multifocal contact lenses follow markedly different design considerations. Known contact lenses with more than a single power are, generally, of three types: simultaneous vision bifocal lenses, simultaneous vision multifocal lenses and diffractive lenses based on the Fresnel zone plate concept. A simultaneous vision bifocal lens is constructed from two or more concentric rings with alternating powers corresponding to distance and reading visions. The wearer of such a bifocal lens observes a scene through both powers simultaneously, relying on a curious psychological effect to overcome the inevitable blurring of the observed scene. When observing a distant scene, the eye focuses in accordance with the distance vision power, while the near vision induces a foggy or hazy background, which, for many individuals, is not objectionable. A simultaneous multifocal contact lens has a progressive radial change in power.
Diffractive lenses, in general, employ a different effect, as compared to the refractive lenses. Diffraction is a phenomenon that occurs when an electromagnetic wave, such as light, encounters an obstacle and propagates non-linearly. This ability to "bend" a part of a light beam is the basic property used to realize any diffractive lens. Diffractive lenses are lightweight substitutes for the conventional refractive lenses used in monochromatic applications.
As for the diffractive contact lens based on the Fresnel zone plate, it can perform just like a refractive lens. As schematically illustrated in FIG. 3, the Fresnel zone plate comprises a circular concentric phase grating PG with a pitch varying quadratically with the distance from the center C.sub.0.
FIG. 4 illustrates a profile 10 of a phase grating utilizing the Fresnel zone plate. For example, the profile 10 is designed such that a single diffraction order, for example "-1", survives. It is understood that with the grating pitch becoming smaller with the radial distance, the diffraction angle, or beam bending effect, grows proportionally, and an incident collimated light beam becomes focused on a point, which is the typical behavior of a lens. This is the physical principle on which the diffractive contact lens relies to achieve its aim.
A multifocal lens can be produced from the diffractive lens by forming a radial grating having several orders, usually two or three, on a substrate shaped like a normal spherical or toric lens. Such a tri-focal diffractive lens is disclosed, for example, in U.S. Pat. No. 5,760,871.
FIG. 5 illustrates the main operational principles of a multifocal diffractive lens 12 comprising a radial grating 14 formed on a spherical substrate 16. The lens 12 focuses images from infinity 18A, from a middle distance 18B and from a reading distance 18C onto an imaging surface, i.e., a retinal surface 20 of the eye. The zero order has the power of the lens substrate 16, which would be observed in the absence of the grating 14. Positive (+1) and negative (-1) diffraction orders produce high and low powers of the lens 12, respectively. The power difference between the positive and the negative orders is referred to as the power addition of the lens.
The advantages of a diffractive lens are associated with the following. The selection of correct design parameters of the diffractive lens allows for minimizing its chromatic aberrations, which are present in any refractive lens. Indeed, the angle of refraction and diffraction angle change with the light wavelength in opposite directions, and the combination of these two parameters produces a near achromatic lens. Other advantages of the diffractive lens are the absence of distortion and the availability of the entire lens area for far, intermediate and near vision.
However, the diffraction lens suffers from drawbacks associated with the fact that only a small percentage of light (about 30-40%) is available for each focusing range, and with the unavoidable image blurring. A technique disclosed in the above patent is aimed at increasing the percentage of incident light generated through each diffraction order. To this end, the diffraction grating has a specific pattern (e.g., trapezoidal protrusions and trapezoidal recesses) with the phase difference between light passing through protrusions and recesses (optical height) substantially less than the half of an average optical wavelength viewed with the lens.